GABAA receptor (γ-aminobutyric acidA) is a pentameric protein which forms a membrane ion channel. GABAA receptor is implicated in the regulation of sedation, anxiety, muscle tone, epileptogenic activity and memory functions. These actions are due to defined subunits of GABAA receptor, particularly the α1-and α2-subunits.
Sedation is modulated by the α1-subunit. Zolpidem is characterized by a high affinity for the α1-receptors and its sedative and hypnotic action is mediated by these receptors in vivo. Similarly, the hypnotic action of zaleplon is also mediated by the α1-receptors.
The anxiolytic action of diazepam is mediated by the enhancement of GABAergic transmission in a population of neurons expressing the α2-receptors. This indicates that the α2-receptors are highly specific targets for the treatment of anxiety.
Muscle relaxation in diazepam is mainly mediated by α2-receptors, since these receptors exhibit a highly specific expression in spinal cord.
The anticonvulsant effect of diazepam is partly due to α1-receptors. In diazepam, a memory-impairing compound, anterograde amnesia is mediated by α1-receptors.
GABAA receptor and its α1-and α2-subunits have been widely reviewed by H. M öhler et al. (J. Pharmacol. Exp. Ther., 300, 2-8, 2002); H. M öhler et al. (Curr. Opin. Pharmacol., 1, 22-25, 2001); U. Rudolph et al. (Nature, 401, 796-800, 1999); and D. J. Nutt et al. (Br. J. Psychiatry, 179, 390-396, 2001).
Diazepam and other classical benzodiazepines are extensively used as anxiolytic agents, hypnotic agents, anticonvulsants and muscle relaxants. Their side effects include anterograde amnesia, decrease in motor activity and potentiation of ethanol effects.
Insomnia is a highly prevalent disease. Its chronicity affects 10% of the population and 30% when transitory insomnia is computed as well. Insomnia describes the trouble in getting to sleep or staying asleep and is associated with next-day hangover effects such as weariness, lack of energy, low concentration and irritability. The social and health impact of this complaint is important and results in evident socioeconomic repercussions.
Pharmacological therapy in the management of insomnia firstly included barbiturates and chloral hydrate, but these drugs elicit numerous known adverse effects, for example, overdose toxicity, metabolic induction, and enhanced dependence and tolerance. In addition, they affect the architecture of sleep by decreasing above all the duration and the number of REM sleep stages. Later, benzodiazepines meant an important therapeutic advance because of their lower toxicity, but they still showed serious problems of dependence, muscle relaxation, amnesia and rebound insomnia following discontinuation of medication.
The latest known therapeutic approach has been the introduction of non-benzodiazepine hypnotics, such as pyrrolo[3,4-b]pyrazines (zopiclone), imidazo[1,2-a]pyridines (zolpidem) and, finally, pyrazolo[1,5-a]pyrimidines (zaleplon). Later, two new pyrazolo[1,5-a]pyrimidines, indiplon and ocinaplon, have entered into development, the latter with rather anxiolytic action. All these compounds show a rapid sleep induction and have less next-day hangover effects, lower potential for abuse and lower risk of rebound insomnia than benzodiazepines. The mechanism of action of these compounds is the alosteric activation of GABAA receptor through its binding to benzodiazepine binding site (C. F. P. George, The Lancet, 358, 1623-1626, 2001). While benzodiazepines are unspecific ligands at GABAA receptor binding site, zolpidem and zaleplon show a greater selectivity for α1-subunit. Notwithstanding that, these drugs still affect the architecture of sleep and may induce dependence in long-term treatments.
Research for new active compounds in the management of insomnia answers an underlying health need, because even recently introduced hypnotics still affect the architecture of sleep and may induce dependence in long-term treatments.
It is therefore desirable to focus on the development of new hypnotic agents with a lower risk of side effects.